'instanced' type for switching between uniforms and attributes

[Aeva]

This is an example of how the syntax might look:

attribute vec3 position;
instanced mat4 world_matrix;

void main(void) {
  // do some stuff
}

And this is an example of how the compiler output might look:

attribute vec3 position;

uniform bool _instanced_render;
attribute vec4 _inst_attra_world_matrix;
attribute vec4 _inst_attrb_world_matrix;
attribute vec4 _inst_attrc_world_matrix;
attribute vec4 _inst_attrd_world_matrix;
uniform mat4 _inst_uni_world_matrix;
mat4 world_matrix;

void main(void) {
  if (_instanced_render) {
    world_matrix[0] = _inst_attra_world_matrix;
    world_matrix[1] = _inst_attrb_world_matrix;
    world_matrix[2] = _inst_attrc_world_matrix;
    world_matrix[3] = _inst_attrd_world_matrix;
  }
  else {
    world_matrix = _inst_uni_world_matrix;
  }

  // do some stuff
}

---

The variable _instanced_render would be used to control when instancing is active, in which case the instanced variables would be populated from their respective attribute (or attributes, in the case of mat3 and mat4s). When it is false, the respective uniforms are referenced instead.

The generated uniforms would alias back to the expected value so that the application interface stays sensible. The attributes should not be handled per object, but rather be disabled by default and explicitly enabled when instancing is needed and be disabled afterwards - or there could be some kind of explicit mechanism for enabling instancing vis disabling it to eliminate state changes.

[Aeva]

Adding this feature should be fairly easy, especially because of the refactoring done as part of #196. The steps are something like so:

• Add a new mode for "instanced" here
• Update this code so that it doesn't assume that all macros have a fake mode in front of them
• Add a new macro to ast.macros.js
• Add metadata onto the ast tree object returned by the compiler
• Use that metadata to add aliases and the likes to the final program object.

[Aeva]

I came up with a better keyword for this while figuring out what to do with #203:

attribute/uniform vec3 position;

I like this version because syntax highlighting still works in github :)

[Aeva]

Probably going to go with this for now, because it is more compact:

in/uniform vec3 position;

[Aeva]

Started working on this in the "instancing" branch from "fast_graph".

Things to do next:

• [x] macro to prepend boilerplate into a shader's main method to allow generic and seamless access
• [x] uniform aliases for instancing switch & non-instanced uniform
• [x] instancing switch default value should always be false, not null
• [x] ~binding context limitations? (or just test it to make sure it works ok)~ the virtual uniforms will always be in the GraphNode binding context. In/uniforms can be defined in a binding context, though it won't do anything.
• [x] add a field to the final shader program object to provide metadata for what can be instanced
• [x] fix broken shaders (eg transition effects, or anything that redundantly defines globals via imports)
• [x] downgrade in/uniform to uniform when instancing isn't available

Actually setting up the buffers for instancing is the scope of another issue (possibly #246).